The principal investigator has completed 6 years of postdoctoral training at the National Institutes of Health and is now dedicated to establishing an independent position in academic research at the Johns Hopkins University School of Medicine. Cancer represents one of the leading causes of death in the United States. To date, CD8+ T cells have been the primary focus in the immunotherapy of cancer because most tumors predominantly express MHCclass I molecules. However, when tumors lose class I expression, they are no longer recognized by CD8+ T cells. This is where immunotherapy fails. Therefore, it is prudent to find alternative means that can mediate long-lived cancer treatment in the absence of CD8+ T cells. It has been shown in mice that CD4+ T cells can reject both class I and Class II negative tumors independent of CD8+ T cells. However, the mechanism by which this occurs is poorly understood. Our goal is to understand how CD4+ T cells can reject established cancer and to develop novel translational cancer immunotherapies. Therefore, we have developed a new T cell receptor transgenic mouse that produces MHC class II restricted CD4+ T cells that recognize a tumor/self-antigen that is also a melanocyte differentiation antigen, called tyrosinase related protein-1 (tyrp-1). We will use CD4+ transgenic T cells directed against B16 melanoma, a poorly immunogenic tumor, to study the requirements to treat established cancer in mice. Thus far we have found that antigen-specific CD4+ T cells can cause the regression of established tumors and cause autoimmune vitiligo through a CD8+ T cell independent mechanism. Our hypothesis is that CD4+ T cells may be activating cells of the innate immune system by upregulating stimulatory ligands or downregulating inhibitory ligands on tumor and self- tissues. This process allows recognition of tumors in an antigen-specific fashion. The specific aims are (1) to identify the cellular populations that CD4+ T cells cooperate with in vivo to treat established tumors in lymphopenic mice, (2) to determine if CD4+ T cells change the expression pattern of inhibitory and stimulatory molecules on tumor for cells of the innate immune system, and (3) to understand the regulatory mechanisms which prevent tumor therapy and develop therapies to overcome self-tolerance without inducing lymphopenia. In summary, CD4+ T cells represent an attractive alternative for immunotherapy because they can mediate tumor rejection in multiple ways.